In electrographic imaging a latent image of electric charge is formed on a surface of a carrier sheet. Toner particles that are attracted to the charge are applied to the surface of the carrier sheet to render the latent image visible. The toned image is fixed, either by fusing the toner particles to the surface of the carrier sheet, or by first transferring the toned image to a receptor and fusing, or otherwise permanently affixing, the particles to the receptor.
The latent image is produced by imagewise deposition of electrical charge onto the carrier surface. Typically, charged styli, arranged in linear arrays across the width of a moving dielectric surface, are used to create the latent image. Such processes are disclosed, for example, in Helmberger, U.S. Pat. No. 4,007,489; Doggett, U.S. Pat. No. 4,731,542; and St. John, U.S. Pat. No. 4,569,584.
An electrographic imaging element requires a conductive layer. The conductive layers may be metallic, such as when a sheet of metal is used as a substrate for the imaging element. Or it may be a conductive coating on an otherwise non-conducting substrate, such as a polyethylene terephthalate film coated with a conductive metal oxide, such as tin oxide, or a paper sheet bearing a conductive coating.
It is well known that both bulk and surface electrical conductivity can be imparted to many types of materials, especially polymeric materials, by the incorporation of ionic substances, such as monomeric or polymeric quaternary ammonium salts. Low concentrations of ionic additives, or of hydroscopic compounds such as polyglycol ethers or amines, can provide antistatic properties, i.e, surface resistivity of about 10.sup.9 -10.sup.10 .OMEGA./.quadrature.. Larger concentrations of quaternary salts can afford surface resistivities as low as 10.sup.2 -10.sup.5 .OMEGA./.quadrature..
There are, however, significant problems associated with such conductivizing agents. Low molecular weight quaternary salts and hydroscopic additives migrate from the bulk of the host material to the surface. As the conductivizing agent diffuses to the surface the surface properties may vary with time. Because the conductivizing agent is not bound to the surface, contact with other materials can remove it.
Although these problems are solved by the use of polymeric conductivizing agents as bulk additives, there are significant problems associated with these materials as well. Use of the widely used quaternary derivatives of polystyrene and the cyclopolymer derived from dimethyldiallylammonium chloride (DMDAAC) is limited both by their water and alcohol solubility and by their tendency to become soft, tacky, and fragile at high relative humidities, where they exhibit their maximum conductivity.
Polymeric quaternary salts are also immiscible with most other polymers, which limits there usefulness as bulk additives. Because phase separation may occur during the coating process, it is difficult to produce homogenous coatings with these materials.
As described in Shay, U.S. Pat. Nos. 4,322,331 and 4,420,541, some of these problems can be overcome by addition of polymerizable quaternary ammonium monomers to radiation polymerizable compositions to produce a conductive cross-linked copolymer. However, high levels of quaternary ammonium monomers are required to produce conductivities that are useful for the production of electrographic imaging elements. Thus, the polymers formed by polymerization of these compositions tend to be hydroscopic and produce coatings that are soft and tacky. These coatings generally do not form acceptable electrographic element. Therefore, a need exists for polymerizable compositions that will produce conductive polymeric materials with apparent surface resistivities of 1.times.10.sup.4 to 1.times.10.sup.7 .OMEGA./.quadrature., yet do not contain so much polymerizable, ethylenically unsaturated ammonium precursor that the coating is soft and tacky.